1. Field of the Invention
The present invention relates to a method of preparing chiral aromatic cyanohydrins.
2. Description of the Prior Art
Chiral aromatic cyanohydrins, including optically-active alpha-hydroxy benzonitriles are known in the art and are of interest, per se, and as intermediates, e.g. to esters. In pyrethroid esters, those having an alpha-S-alpha-hydroxynitrile moiety coupled with the appropriate pyrethroid acid usually have the highest pesticidal activity. However, such alpha-S-alpha-hydroxynitriles have not been particularly easy to obtain in the past because they were usually prepared by resolution.
Asymmetric synthesis of R-mandelonitrile by addition of hydrogen cyanide to benzaldehyde in the presence of a synthetic dipeptide catalyst is known in the art, as in Oku, Jun-ichi and Shohei Inoue, J.C.S. Chem. Comm., pages 229-230 (1981), and other Oku publications where, e.g., cyclo(L-phenylalanyl-L-histidine) containing 1/2 mole of water of crystallization was used. However, it has been found that the process with cyclo(L-phenylalanyl-L-histidine) and cyclo(D-phenylalanyl-D-histidine) are not necessarily satisfactory or optimal for the preparation of chiral cyanohydrins because of problems with the extent of the induction time for the process and the associated problem of controlling heat released by addition of cyanide ions. Applicants have now found that these problems can be avoided while also increasing further the enantiomeric selectivity of the chiral cyanohydrin product.